Even more problems of this sort arise when we consider problems of transportability or external validity—assessing whether an experimental result will still be valid when transported to a different environment that may differ in several key ways from the one studied. This more ambitious set of questions touches on the heart of scientific methodology, for there is no science without generalization. Yet the question of generalization has been lingering for at least two centuries, without an iota of progress. The tools for producing a solution were simply not available.
